The proposed research will attempt to replicate and extend previous preliminary observations which suggest that immune processes are subject to behavioral conditioning. Using the illness-induced taste aversion paradigm, rats and/or mice will be conditioned by pairing a distinctively flavored drinking solution (the conditioned stimulus) with cyclophosphamide or other immunosuppressive agents (the unconditioned stimulus). After conditioning, all animals will be injected with antigen (e.g., sheep erythrocytes). Antibody titers will be measured after antigen administration in conditioned animals that are again exposed to the conditioned stimulus or that receive no further experimental treatment, and in nonconditioned and placebo-treated groups. The primary focus of this initial research will be to study systematically those parameters of the conditioning process and those parameters of the immune response which are optimal for inducing conditioned immunosuppression. Proposed studies will also examine the extent to which behaviorally conditioned immunosuppression might be mediated by specific or nonspecific activation of the pituitary-adrenal axis.